Infrared spectroscopy through the orientational phase transition in fullerene films

Abstract

Infrared-absorption studies of thin fullerene films as a function of temperature reveal a splitting of an absorption line at 245+3 K. Another absorption line narrows and its frequency shifts without splitting.These observations support earlier neutronand x-ray-di6raction findings of an orientational phase transformation at 250 K in bulk samples. We provide an explanation based on an accepted crystal structure of C60.The recent discovery of the eScient synthesis of C6p (Ref. 1) has resulted in a fiurry of experimental and theoretical investigations of this highly symmetric cage structure. In addition, observations of photoconductivity and superconductivity ' in doped C6p have focused our attention on the electronic structure and temperature dependence of structural properties of this unusual material. In this paper we report temperature-dependent infrared (IR) spectra of thin films of C6o from room temperature to 10 K. We have studied two of the four IR-active modes. One of the IR absorptions at 1183 cm ' narrows and shifts slightly toward higher energies while the other mode at 1429 cm ' splits at approximately 245 K, and becomes better resolved as the temperature is further lowered to 10 K. We offer an explanation based on a model of the vibrational modes of C60 (Ref. 5) and proposed crystal structures.' These observations are consistent with the recently reported orientational ordering in bulk C60 studied using ' C NMR (Ref. 11 and 12) andx-ray and neutron diffraction.C6p was prepared in an arc and purified chromatographically to remove impurities including C7p

Insulating and conducting phases ofRbC60

“…According to the NMR results [7] the rapid rotation of the C 60 molecules (characteristic of the high temperature phase) stops below the phase transition. In pristine C 60 the single F 1u (4) line splits to three lines (separated by ∼ 3cm −1 ) when the molecular rotation ceases [16]. The doublet seen in our measurement can be due to stronger crystal fields with appropriate symmetry, related to the new low temperature structure [14], such that two of the three F 1u (4) modes are left degenerate.…”

“…These modes remain visible in both the high temperature and the slow cooled metallic phases. The F 1u (3) mode around 1183cm −1 seems to be split, but this mode is not sensitive to doping [10,11] or crystal field effects [16].…”

Magnetic Susceptibility of Molecular Carbon: Nanotubes and Fullerite

“…Other probes that are sensitive to the intramolecular spectrum do not show as marked an effect. In infrared spectroscopy studies (21) there is no sharp feature at T, in vibrational-mode absorption lines at 1 183 and 1430 cm-l , beyond the more gradual changes associated with rotational freezing. Similarly, in Raman spectroscopy (22), changes in vibrational modes at 1430 and 1555 cm-' occur in the vicinity of T,, but these changes are spread over at least 40 K, in contrast to the transition width of -0.5 K in the present measurement.…”

Jahn-Teller distortion in Cs4C60 studied by vibrational spectroscopy

“…It is tempting to offer an explanation of the peak height of the weaker lines (Fig. 2, right panel) based on this theory, however, since the allowed 572 cm" 1 peak is broadening rather than narrowing in this region (similarly to pristine C 60 around the orientational phase transition [12]), we believe that all lines reflect the symmetry change and internal dynamics of the C^ã nions rather than that of the environment. Local symmetry lowering without long-range reorientation could also explain the lack of thermal effects around the transitions.…”